In a two-motor type hybrid vehicle or a hybrid drive unit, a power of an internal combustion engine (i.e., an engine) is distributed by a power distribution device composed mainly of a differential mechanism to the first motor having a generating function and to an output member, and an electric power generated by the first motor is delivered to the second motor connected with the output member to operate the second motor. Thus, in the two-motor type hybrid vehicle, the mechanical power of the engine is partially delivered to the output member through the power distribution mechanism, and the remaining power of the engine is once converted into an electric power and then converted into the mechanical power again to be delivered to the output member. Therefore, an engine speed can be controlled arbitrarily by the first motor while keeping a drive torque to achieve a required acceleration etc. so that the engine is allowed to be operated at an operating point of optimizing fuel efficiency.
Such advantage is realized by changing the engine speed continuously by the first motor, therefore, the speed ratio of the two-motor type hybrid vehicle is changed substantially continuously as a vehicle having a continuously variable transmission. However, in order to enhance an agility of the vehicle, the drive torque or an engine braking force has to be increased while maintaining the substantial speed ratio. To this end, the engine speed of the conventional hybrid vehicle has been changed stepwise. In order to realize such engine speed control or speed ratio control, a region where the speed ratio is maintained, and a region where the engine speed is changed with a change in a vehicle speed are determined in advance for different driving conditions represented by the vehicle speed or an opening degree of an accelerator. In the conventional hybrid vehicle, therefore, a target engine speed (i.e., a speed ratio) is changed as the driving condition enters into the above-explained region or the driving condition is changed beyond the above-explained region. Otherwise, the target engine speed (i.e., the speed ratio) is changed as a speed ratio is changed by a manual operation.
In the above-explained hybrid vehicle, the engine speed is basically controlled by the first motor. That is, the first motor is required to establish a large positive or negative torque to change the engine speed stepwise in response to a speed change demand. For this purpose, an electric storage device such as a battery has to discharge an electric power, and charged with the electric power. However, such charging and discharging of the storage device may be restricted depending on the condition. In addition, an inertia torque is generated in accordance with an inertia moment as a result of changing the engine speed, and such inertia torque may become load on the storage device and may cause a control delay.
In the prior art, various kinds of devices for preventing an over discharge and an overcharge of the electric storage device used in the hybrid vehicle have been proposed. Also, various kinds of devices for controlling the speed ratio and the engine speed taking into account the inertia torque of the engine have been proposed. For example, Japanese Patent Laid-Open No. 2008-179291 describes a power output device adapted to quickly execute a upshifting manually in the two-motor type hybrid vehicle. As described, the engine speed is controlled by a generator, and a torque of the generator has to be increased to lower the engine speed quickly or stepwise. However, the torque of the generator may be limited depending on a charging capacity of the electric storage device. According to the teachings of Japanese Patent Laid-Open No. 2008-179291, therefore, a required power for the engine is limited in accordance with inertia energy resulting from lowering the engine speed.
When changing the engine speed, a torque is generated or required according to the inertia moment. Therefore, the device taught by Japanese Patent Laid-Open No. 2008-179291 carries out the controls taking into account the inertia energy. Likewise, Japanese Patent Laid-Open No. 2011-37400 describes a method for controlling a motor torque taking account of an inertia torque of the engine, in the process of stopping the engine by stopping a fuel injection to the engine. When the generator is activated to stop a rotation of the engine, the torque of the generator is added to the driving torque for propelling the vehicle more than a little. According to the teachings of Japanese Patent Laid-Open No. 2011-37400, therefore, a torque obtained by subtracting a torque of the generator for stopping the engine from a request torque requested in travel is outputted from a motor connected with an output shaft. In addition, the inertia torque is generated when lowering the engine speed to stop the engine. According to the teachings of Japanese Patent Laid-Open No. 2011-37400, moreover, a torque obtained by subtracting the inertia torque of the engine and the torque of the generator is outputted by the motor after the lapse of the predetermined time.
If a response delay during changing the engine speed is compensated by the motor, a generation amount of the motor is increased and this may cause an overcharging of a battery. Otherwise, an over discharging may be induced to increase the motor torque. In order to prevent such disadvantages, in a hybrid vehicle described in Japanese Patent Laid-Open No. 2011-105240, a target drive power is calculated based on an accelerator opening degree and a vehicle speed obtained preset time earlier, and a target motor generator power is calculated based on the target drive power thus calculated.
Japanese Patent Laid-Open No. 2005-315084 also describes a control example to deal with the response delay during changing the engine speed or an engine output. According to the teachings of Japanese Patent Laid-Open No. 2005-315084, a timing to start a downshifting for establishing an engine braking force is accelerated taking into consideration a delay in controls of an engine and an engagement of a clutch.
Meanwhile, Japanese Patent Laid-Open No. 2006-63865 describes another power output device configured to estimate an engine torque while changing a response speed of the control in accordance with an engine speed.
In turn, Japanese Patent Laid-Open No. 2000-87774 describes a power output apparatus and a control method for a hybrid vehicle. According to the teachings of Japanese Patent Laid-Open No. 2000-87774, an operation line lower in torque than the maximum torque line is adapted as a reference operation line. When an abrupt acceleration is required, the engine torque is increased to the maximum torque line, and then an engine speed is increased to a speed at an intersection of the operating line in a steady state and the equivalent output line of the abrupt acceleration.
As described in Japanese Patent Laid-Open No. 2008-179291, the power generation amount may be reduced by lowering the engine output during lowering the engine speed while taking account of the inertia energy of the engine so as to reduce the torque to be established by the generator. However, it may take time to consume the inertia energy resulting from lowering the engine speed. Therefore, a reduction in the engine speed, that is, the speed change operation may be delayed.
According to the teachings of Japanese Patent Laid-Open No. 2011-37400, the motor torque is controlled taking account of the inertia torque of the engine so that the drive torque is stabilized in the process of stopping the engine. However, according to the method taught by Japanese Patent Laid-Open No. 2011-37400, the motor torque is controlled only until the inertia torque resulting from changing the engine speed disappears. That is, a length of time until a disappearance of the inertia torque cannot be shortened. Therefore, the reduction in the engine speed, that is, the speed change operation may also be delayed.
In turn, according to the teachings of Japanese Patent Laid-Open No. 2011-105240, the target drive power obtained based on the values of preset time earlier is used. Therefore, it is unnecessary to increase the generation amount when an accelerator pedal is returned to cutoff power so that the vehicle speed and the engine speed are lowered. However, in the hybrid vehicle taught by Japanese Patent Laid-Open No. 2011-105240, the engine speed is not lowered aggressively. This means that the control response to change the engine speed may not be improved. Such technical problem may also not be solved by the teachings of Japanese Patent Laid-Opens Nos. 2005-315084 and 2006-63865.
According to the teachings of Japanese Patent Laid-Open No. 2000-87774, the engine speed is changed continuously to control the engine torque when an abrupt acceleration is required. Specifically, the engine torque is increased first of all, and then, the engine speed is changed along the equivalent output line for the case that the abrupt acceleration is required. Therefore, an increase in the engine speed is delayed inevitably. In addition, since the control is carried out without taking into consideration the inertia moment of the engine, the response delay may be worsened when changing the engine speed stepwise by a manual operation.